Field of the Invention
The present invention relates to a bias circuit that supplies a bias voltage to a microphone.
Description of the Related Art
Electronic devices having a recording function or a telephone calling function such as camcorders, digital still cameras, cellular phone terminals, tablet terminals, etc., each mount a microphone, a bias circuit that biases the microphone, and an amplifier that amplifies an electric signal output from the microphone.
FIG. 1 is a circuit diagram showing a bias circuit for a microphone investigated by the present inventor. The bias circuit 30 includes a voltage source 32 and a variable gain amplifier 34, and generates a bias voltage VBIAS to be supplied to the microphone 2. The output terminal of the bias circuit 30 is connected to a capacitor C1 for phase compensation. The bias voltage VBIAS thus generated is supplied to the microphone 2 via a resistor R1.
The voltage source 32 divides a stabilized voltage VREG so as to generate a reference voltage VREF. The variable gain amplifier 34 is configured as a non-inverting amplifier. The variable gain amplifier 34 amplifies the reference voltage VREF with a given gain so as to output the bias voltage VBIAS. The suitable level of the bias voltage VBIAS varies according to the kind of microphone 2. Thus, in order to provide the bias voltage VBIAS suitable for each microphone 2, the gain of the variable gain amplifier 34 is adjusted.
The microphone 2 converts an input acoustic signal into an electric signal. The AC component, which is obtained by removing the DC component from the electric signal, is input to the microphone amplifier 40 via a capacitor C2 for DC blocking. The microphone amplifier 40 amplifies such a weak electric signal. An A/D converter 42 converts the output of the microphone amplifier 40 into a digital signal. The bias circuit 30 shown in FIG. 1 cannot be recognized as a known technique. The microphone amplifier 40 and the A/D converter 42 are integrated as an audio interface circuit 4r together with the bias circuit 30.
As a result obtained by investigating the bias circuit 30 shown in FIG. 1, the present inventor has come to recognize the following problem. An unshown digital circuit is mounted as a downstream stage of the A/D converter 42. High-frequency noise that occurs due to a clock signal generated in the digital circuit contaminates the bias circuit 30 via the ground line. Specifically, as shown in FIG. 1, such noise contaminates the bias voltage VBIAS from the ground via contamination paths (i) through (iii). The bias voltage VBIAS noise is input to the microphone amplifier 40 via the capacitor C2, leading to a problem of degraded sound quality.
In particular, the noise contamination via the paths (i) and (ii) is amplified by the variable gain amplifier 34. Thus, such noise becomes a factor degrading the sound quality.